Carbon dioxide mediated dissolution of Ca-feldspar: implications for silicate weathering

Experimental studies on the dissolution kinetics of anorthite under N-2(g) and CO2(g) atmospheres at 25 degrees C in electrolyte solutions, pit range 5.5 < pH < 8.5, have been performed using laboratory flow-through reactors. Aluminum release from anorthite is accelerated in the neutral to near-basi c pH region. Because Al is the slowest dissolving network-forming cation un der these conditions, we propose that accelerated Al release corresponds to a long-term acceleration of anorthite dissolution. The rate of Al release from anorthite correlates with solution concentration of carbonate ion acco rding to a fractional-order empirical rate law; k = (1.1 +/- 1) x 10(-7) mo l(0.76) m(-1.52) h(-1): Rate = k[CO32-](0.24) We propose a two-step reaction mechanism where inorganic carbon is rapidly adsorbed, forming a reactive bi-dentate surface Al-carbonate complex that i s released to solution in a much slower, irreversible step. The rate expres sion formally derived from this dissolution mechanism is consistent with th e observed dependence of Al release rate on the master variables PCO2 and p H. A comparison with published data on the weathering kinetics of plagiocla se in solutions of oxalic acid shows that the reactivity of carbonate is si milar to that of the organic ligand oxalate. Because of pH effects on the s peciation of these ligands in solution and on mineral surfaces, carbonate p romoted weathering is especially important at near-neutral and basic pH. We propose that the generally termed "carbonation weathering", the effect of elevated subsurface PCO2 to accelerate rock weathering through suppression of pH, be extended to include the speciation and reactivity of inorganic ca rbon at neutral and near-basic pH. (C) 2000 Elsevier Science B.V. All right s reserved.